(1) Field of the Invention
The present invention relates to an epoxy modified polysiloxane useful for sealing light emitting devices which exhibit excellent light resistance and excellent adhesiveness.
(2) Description of the Related Art
Recently, for blue light emitting diodes (hereinafter abbreviated as LED) and ultraviolet light emitting diodes having a wavelength of 250 nm to 550 nm, high brightness products using a GaN type compound semiconductor such as GaN, GaAlN, InGaN and InAlGaN have become available, and as a result, the formation of high definition full color images has been achieved in combination with red LEDs and green LEDs. In addition, by combining a blue LED or a ultraviolet LED with fluorescent materials, white LEDs have been obtained and the demands for backlights of liquid crystal displays and general lighting are expected to increase.
Conventionally, as a sealing agent for light emitting devices such as red or green LEDs, epoxy resins using an acid anhydride type curing agent have been used because of the high adhesiveness and the excellent mechanical durability they provide. However, such epoxy resin sealing agents have had the problem that the agents have a low light transmittance against blue to ultraviolet light in short wavelength regions and cause coloring due to photodegradation which has resulted in low light resistance against light in short wavelength regions. To overcome this defect, the use of an alicyclic hydrocarbon epoxy resin as a main component has been suggested in some publications (JP-A-11-274571, JP-A-2000-196151 and JP-A-2002-226551). The level of the light resistance, however, is still insufficient and further improvement is required.
Studies of the present inventors have revealed a tendency for an alicyclic hydrocarbon epoxy resin to have decreased adhesiveness compared to a corresponding aromatic hydrocarbon epoxy resin and the adhesiveness and the light resistance have a trade-off (opposing) relationship.
On the other hand, as a sealing agent that has high light resistance against light in short wavelength regions, silicone resins have been used. Generally, silicone resin sealing agents have low adhesiveness, and light emitting devices which frequently have a remarkable heat shock because of the on-off of light emission and no light emission or the conditions of installation in the apparatus are often associated with problems such as peeling between the device and the sealing agent due to the differences in the coefficient of thermal expansion, humidity invasion due to the peeling, and deterioration of the device. For this reason, there are proposals for using a mixture of silicone resin and an epoxy resin, which together are excellent in adhesiveness (JP-A-2002-324920). The properties required for adhering the device and the sealing agent naturally depend on the purpose of use, but, unfortunately, the adhesiveness is still insufficient for such use as white LEDs, backlights of liquid crystal displays and general lighting where the devices are exposed to outside air or subjected to long and continuous use. Further improvement of the adhesiveness is therefore strongly required.
Thus, to what level a modified silicone, in which an alicyclic hydrocarbon epoxy group is introduced into the side chain of the silicone skeleton, which is a candidate material having both the excellent adhesiveness and the durability of an epoxy and the excellent light resistance of a silicone, can achieve both the adhesiveness and the light resistance required has been open to investigation.
A modified silicone having a similar structure and containing an aromatic hydrocarbon epoxy group has been disclosed in, for example, JP-A-2-028211, JP-A-4-034908 and Japanese Patent No. 3283606. In addition, those skilled in the art have had the common knowledge that replacement of the aromatic hydrocarbon moiety with an alicyclic hydrocarbon improves the light resistance as disclosed in JP-A-11-274571, JP-A-2000-196151 and JP-A-2002-226551. Therefore, the essential issue of the investigation is naturally the level of the adhesiveness.
As a result of detailed studies of the present inventors on a synthesis method and use of modified polysiloxane containing alicyclic hydrocarbon epoxy group, it has been found that a paper describing a method for the synthesis of the above-mentioned compound has already been published by Crivello et al in 1990 (J. V. Crivello and J. L. lee, The Synthesis, Characterization and Photoinitiated Cationic Polymerization of Silicon-Containing Epoxy Resins, Journal of Polymer Science: Part A: Polymer Chemistry, United States, John Wiley & Sons, Inc. (1990), Vol. 28, p. 479-503) and that there are also descriptions of the substance, i.e., an alicyclic hydrocarbon epoxy modified silicone itself in some specifications.
The substance is mostly used as, for example, a peeling agent or an anti-adhesive coating as shown in JP-A-2003-327664, JP-A-2001-220512, Japanese Patent No. 2623426 and National Publication of International Patent Application No. 2000-516660. Other uses include additives for resins (JP-A-10-152560, JP-A-6-299079 and JP-A-11-256035), gaskets JP-A-2000-508371 and Japanese Patent No. 3192155) and sealing agents for a housing (JP-B-6-017445), which are included in the category of general use of silicone resins.
When the compound is used for the surface treatment of release paper, to decrease adhesiveness, decreased adhesiveness is an essential condition as the word of “release” suggests. In short, the compound has been used for a purpose completely opposite from the purposes of the present invention such as for the sealing of light emitting devices, where to increase adhesiveness, increased adhesiveness is a requirement.
Although adhesiveness may be admittedly decreased to a certain degree, if the adhesiveness is decreased to a level for use as release paper, there has been no other choice, but to believe that the compound cannot be used for the sealing of light emitting devices at all, where a high adhesiveness is required.
Thus, it has been commonly inconceivable for those skilled in the art who can conduct a prior art search and do research and development activities to consider using an alicyclic hydrocarbon epoxy modified silicone, a characteristic which is low adhesiveness, for the purpose of sealing light emitting devices which require a high adhesiveness.